Noob question

[robertjhoag]

Okay, I have heard a number of different explanations of oil vis - eg. 15W 40. Can someone please confirm?

I have always thought that with a multi grade oil, the lower grade number indicated the cold flow rate, meaning the oil flows like a straight 15 viscosity at 40*C, and when it warms it "thickens" via VII to flow like a straight 40 vis at 100*C.

One person insisted that the oil always flows as a 15 vis no matter the temp, but the additive pack PROTECTS like a 40 vis at higher temp. Supposedly explains the pressure drop when warm.

Another tells me that the oil flows when cold as a 55 viscosity (15+40) and as it warms, begins flowing as a 40 vis, also explaining the pressure drop when warm.

Both friends insist that the higher pressure cold and pressure drop when hot is the reason their explanation is the correct one

Can someone tell me who is right?

 

[Johnny]

Originally Posted By: robertjhoag
Okay, I have heard a number of different explanations of oil vis - eg. 15W 40. Can someone please confirm?

I have always thought that with a multi grade oil, the lower grade number indicated the cold flow rate, meaning the oil flows like a straight 15 viscosity at 40*C, and when it warms it "thickens" via VII to flow like a straight 40 vis at 100*C.



This is closer to the correct answer. And, friend number two is totally off base.

 

[Quattro Pete]

Originally Posted By: robertjhoag

I have always thought that with a multi grade oil, the lower grade number indicated the cold flow rate, meaning the oil flows like a straight 15 viscosity at 40*C, and when it warms it "thickens" via VII to flow like a straight 40 vis at 100*C.

No oil thickens as it warms up. In fact, just the opposite. The hotter it gets, the thinner an oil becomes.

Look at it from the other end: When at operating temperature (say 100 deg. C), both a straight 40 and a 15w40 will be just as thin, and that's as thin as they'll ever get in normal engine operating environment. The difference is, when the oil cools, the straight 40 will thicken more than the 15w40. They'll both still become more thick, it's just that the 15w40 will not thicken as much, thus providing better cold startup lubrication.

 

[bepperb]

The second relates to a range of viscosities that the oil can be in at operating temp.

The first, or w number, is the lowest test is passes for winter performance, where the oil needs to pass an orifice at a certain volume under a certain pressure.

 

[ARCOgraphite]

In the "OLD" days (where I come from) You ran and had only SAE single grade oils. You typically ran a SAE 30 or 40 in the summer, and in colder climates up North America, would run a SAE 20 in the winter. So when multigrade were 'invented' and marketed, the W numerber represented the SAE grade that you would traditionally choose to run in the winter, and the right hand number was the grade you ran in the summer - and actually representative of the SAE grade viscosity at engine coolant boiling.

The lower left hand # on WINTER multigrades grades means the oil flows/shears/pumps SIMILAR to what the lower # single grade would shear/flow/pump WHEN chilled to the winter test temp (much lower than 40 deg c),

Said another way, the oil would THICKEN more moderately; similar to how the lower # represented W grade would thicken when cooled. Multigrades all thicken greatly, the curve is less steep on the multigrades, and the LOWER the first W number the less steep the viscosity curve up to the asymptotic margin of solidification - way down below 0. From cold to hot , left to right the curve looks like this: \ with increasing temp on X-axis, and increasing viscosity plotted in the Y.

 

[ARCOgraphite]

QuatP's answer is the least confusing to a layperson so far

 

[Kaboomba]

The purpose of the multi-weight is to try to keep the oil viscosity in a certain sweet spot that the engine likes, throughout the range of temperatures the oil experiences. i.e. you don't want to pump crisco through your rod journals when you start up and then pump lighter fluid through them when you are idling in traffic on a 90F day.

A tale of two oils: 5w30 and 10w30

Pretend you start with both oils at normal operating temp, or around 250F. You then put the oils in a freezer and take them down to -60F--

For the 10W: At normal operating temp, it acts like a straight SAE 30 would act at that temp. Then, at some temp "X" it transitions, and starts to behave like a SAE 10 would behave at that temp. It then continues to behave like a 10 as temperature drops, regardless of how low the temp gets.

The 5w30 will act like the SAE 30 at normal operating temp, just like the 10w30. Then it acts like a 10 at X temp and then transition again (in a manner of speaking) to acting like a 5 at some lower temp Y and for all lower temps the way the 10W did at X. These transitions are continuous, and they keep the viscosity of the oil in the ideal range that provides proper film thickness on bearing surfaces.

This is certainly what you want if you live in a place where the seasonal temps swing a lot!

 

[hate2work]

Originally Posted By: ARCOgraphite
QuatP's answer is the least confusing to a layperson so far


And your's is the most confusing LOL

But you still get a "A" for effort

 

[ARCOgraphite]

I thought my first paragraph was anectdotal. At least, it does not have all the CEO-type jargon the Gary A. uses

(said with luv)

 

[lipadj46]

I find it amazing how many people truly believe that oil gets thicker as it gets hotter. I have an oil pan heater and a guy on another forum told me I was going to kill my camshaft because I was starting my truck with thick hot oil.

 

[hate2work]

Originally Posted By: lipadj46
I find it amazing how many people truly believe that oil gets thicker as it gets hotter. I have an oil pan heater and a guy on another forum told me I was going to kill my camshaft because I was starting my truck with thick hot oil.


Tell him to do a very simple in home experiment involving his freezer and his stovetop, that should make it fairly clear to him.

 

[ARCOgraphite]

Like take the leftover burrito filling out of the freezer and heat it refrito caliente en de horno?

 

[m37charlie]

Real definition of 15W40 viscosity:
15W means viscosity 40 means viscosity between 12.5 and 16.3 CSt @ 100 C.
See viscosity chart accessible on home page of this forum.

Charlie

 

[snakyjake]

The thinner oil at cold startup is to get the oil circulating faster and protect the engine. This is because oil is thicker at cold temps than at higher temps. This is the viscosity index to the left.

However, when the engine has heated up, you don't want your oil to get too thin, then it won't protect as good. Therefore you want the oil to thicken as it heats up. This is the viscosity index to the right.

On some oils, you'll also notice the second/right number to be lower to improve fuel mileage. That means the oil isn't as thick at normal temperature, and allows the engine parts to do less work circulating oil.

Jake

 

[Rand]

http://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/question164.htm/printable

start with that one and go through 2,3 and 4